Lubricating oils used for the lubrication of the parts having a wet-type clutch or wet-type brake such as automatic transmissions, tractors and the like are required have properties including good friction characteristic, oxidation stability, corrosion resistance and rustpreventiveness as well as a large torque for power transmission. The friction characteristic here implied is a ratio of the coefficient of static friction and the coefficient of dynamic friction and it is required that this ratio is small and little influenced by the changes in the temperature or in the lapse of time.
In the prior art, there is known such a lubricating oil having a large coefficient of static friction and a good torque for power transmission. This lubricating oil, however, has defects that the friction characteristic thereof is not satisfactory so that a shift of the automatic transmission and the like causes a large shock.
In particular, the trend is more and more outstanding in recent years that automatic transmissions and the like are designed in a decreased size along with the prevalence of compact cars and so-called FF cars (front engine, front wheel driven cars). This trend of automatic transmissions toward compact size promotes the adverse influence that the shift shock is felt sensitively by the driver. Accordingly, it is a technical problem to further improve the friction characteristics in order to improve the comfortableness of driving by decreasing the shift shock.
A proposal has been made accordingly of a lubricating oil by use of a friction modifier with an object of improving the friction characteristics. There is a problem, however, that the lubricating oil containing a friction modifier still has only insufficient friction characteristics along with variation of the friction characteristics by the change in the oil temperature and that the friction characteristics are subject to decrease by the degradation (changes in the lapse of time) of the oil after a long-term service.
Therefore, lubricating oils having good friction characteristics with small changes by oil temperature and in the lapse of time and having a large transmission torque have hitherto not been obtained.
The object of the present invention is to dissolve the above described problems in the prior art by a specific combination of compounds contained in a base oil and to provide a lubricating oil composition with a small shock and a large transmission torque and capable of being satisfactorily used for lubrication of automatic transmissions and the like.